1. Field of the Invention
This invention relates to a process for producing binderless zeolite sieve extrudates by extruding a mixture containing about equal amounts of a zeolite powder, preferably Type A zeolite, a metakaolin clay and a near stoichiometric caustic solution. The clay in the extrudate crystallizes to form a coherent particle that is essentially all zeolite.
2. Description of the Prior Art
Zeolite sieve formates have been made by mixing zeolite particles such as a 4A zeolite with a binder. Conventional formulations contain about 80% zeolite and 20% clay binder. This mixture is then formed into shapes such as beads or extrudates. The problem with this type of forming method is that 20% of the weight of the product is an inert binder which does not contribute to any absorption.
U.S. Pat. No. 3,119,659 to Taggart et al. discloses a process for producing molecular sieve bodies in which a reactive kaolin-type clay is reacted with an alkali metal hydroxide such as sodium hydroxide to form a preformed body which is then reacted with further sodium hydroxide to form a complete zeolite. Taggart et al also illustrate a method where relatively small amounts of a previously formed zeolite are added. However, in the preferred embodiments the Taggart et al. method only adds enough caustic to provide a molar ratio of Na.sub.2 O/SiO.sub.2 in the range of 0.1 to 0.3. Because they do not initially provide enough caustic in the extrudate for complete conversion of the clay, their method requires a post extrusion treatment in which the extrudates are soaked in caustic liquors in both a digestion step and a heated crystallization step.
The Taggart et al. method requires significant time for the digestion step as well as for a crystallization step, both conducted in their examples in the presence of a sodium hydroxide solution having a concentration of sodium hydroxide significantly greater than the 3% sodium hydroxide solution used in the present process. Because all of the clay has not reacted before the extrudate is placed in the digestion liquor, there may be some deterioration in structural rigidity of the extrudates when they are placed in the digestion liquid. The method also requires the extrudates to be dried so the moisture content is less than about 30 weight percent before they can undergo the further sodium hydroxide treatment.
3. Objects of the Invention
It is an object of this invention to produce formed zeolite molecular sieves which contain essentially 100% zeolite with negligible inert binder which have a higher absorption capacity, a faster adsorption rate, and improved selectivity and which are in the form of hard nodules with improved attrition resistant.
It is a further object to produce a zeolite extrudate with a lower dust content.
It is a further object of this invention to form zeolite molecular sieves from a formate containing a substantial amount of metakaolin clay without requiring the extrudates to be immersed in a concentrated caustic solution in a digestion step.
It is a further object to age the clay containing extrudates under dry bulk storage in a container to permit autogeneous heat to develop to crystallize the clay.
It is a further object to produce a small zeolite extrudate having a diameter of about 1/16 to 1/8 inch which contains essentially no inert material.
It is a further object of the invention to obtain calcium exchanged formed zeolites having a nominal pore size of 5 Angstrom units.
These and further objects will become apparent as the description of the invention proceeds.